The distribution of power to support large sets of circuit modules has been challenging because the required voltages and acceptable voltage variances both continue to decrease, while the required peak currents continue to increase. For example, a communications processor currently requires a power source that can provide high current at a low voltage, and a large number of capacitors of multiple values to smooth out high frequency edges and meet surges that fall into different ranges between the high frequency and the response capability of the power supply. Integrated circuit technology has been able to replace the functionality of many devices in digital logic and to produce many analog devices compatibly on a substrate. Power sources have been necessarily handled from outside the chip and this has required lossy and bulky structures to implement power paradigms that maintain low power supply regulated voltage with respect to a common ground. In addition, it is common to have multiple voltage requirements for a single central processor unit. Furthermore, the power is routed by many conductive traces, where even a slight resistance causes large differential voltage drops. No small, efficient power source has been able to be integrated on the chip with the logic. No power source has been able to be produced in the wafer processing cycle without individual manual operations being done to install special elements such as membrane materials. As a result, no simple biasing paradigms have been developed to permit the lower power and higher speeds that can be achieved.
An electrochemical cell is an example of a bias source that may have millimeter or nanometer dimensions. An electrochemical cell includes two half-cells, each of which includes an electrode and a reagent. The reagent in one half-cell undergoes an oxidation reaction at the anode, producing electrons as one reaction product. The reagent in the other half-cell undergoes a reduction reaction at the cathode, consuming electrons as a reactant. Ionic balance between the two half-cells is maintained by an ion-conducting interface between the half-cells. The electron flow from the anode to the cathode will provide an electrical current to an electrical load connected to the two electrodes.
In order for complementary half-cell reactions to take place in an electrochemical cell, ions must travel between the two electrodes. In a conventional electrochemical cell, an ion conducting interface is present between the electrodes. The interface prevents bulk mixing of the reductant and oxidant, but permits ions to flow between the two electrodes. Examples of ion conducting interfaces include a salt bridge, a polymer electrolyte membrane, and an induced dynamic conducting interface (IDCI). Electrochemical cells that include an IDCI are described, for example, in U.S. Pat. No. 6,713,206 B2.
The reagent in the half-cell containing the cathode is an oxidant, since it undergoes a reduction reaction at the cathode. The reagent in the half-cell containing the anode is a reductant, since it undergoes an oxidation reaction at the anode. The electrons produced at the anode can travel through an external circuit to the cathode, where electrons react with the oxidant at the cathode catalyst to produce a reduced product. When the electrochemical cell is a fuel cell, the reductant is a fuel.
Hydrogen, methanol and formic acid have emerged as important fuels for fuel cells, particularly in mobile power and transportation applications. The electrochemical half reactions for a hydrogen fuel cell are listed below.
Anode:2H2 →4H+ + 4e−Cathode:O2 + 4H+ + 4e−→2H2OCell Reaction:2H2 + O2→2H2O
To avoid storage and transportation of hydrogen gas, the hydrogen can be produced by reformation of conventional hydrocarbon fuels. In contrast, direct liquid fuel cells (DLFCs) utilize liquid fuel directly, and do not require a preliminary reformation step of the fuel. As an example, the electrochemical half reactions for a Direct Methanol Fuel Cell (DMFC) in acidic conditions are listed below.
Anode:CH3OH + H2O →CO2 + 6H+ + 6e−Cathode:1.5O2 + 6H+ + 6e−→3H2OCell Reaction:CH3OH + 1.5O2→CO2 + 2H2O
As another example of a DLFC, the electrochemical half reactions for a Formic Acid Fuel Cell (FAFC) in acidic conditions are listed below.
Anode:HC(═O)OH→CO2 + 2H+ + 2e−Cathode:O2 + 2H+ + 2e−→2H2OCell Reaction:HC(═O)OH + O2→CO2 + 2H2O
Several types of fuel cells have been constructed, including polymer electrolyte membrane fuel cells, direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. For a comparison of several fuel cell technologies, see Los Alamos National Laboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power by Sharon Thomas and Marcia Zalbowitz.